Amplitude modulation of optical signals plays an important role in various industrial sectors: e.g. communication, material processing. An extreme form of amplitude modulation is on/off modulation, which is the modulation format, used in pulsed laser material processing. Whereas in communication systems, the achievable modulation bandwidth is the crucial parameter, in laser based material processing it is rather the optical power handling capability. A variety of optical modulation principles exist such as electro-optic, acousto-optic, magneto-optic and modulation schemes such Fabry-Pérots modulators, Mach-Zender interferometers. In some wavelength intervals, however, none of these existing modulation principles and modulation schemes is cost effective, reliable solutions. One of these spectral intervals covers the operation window of CO2 lasers. CO2 lasers are gas lasers emitting mid-infrared radiation of wavelength between 9 and 11 micron. By adapting the constituents of the gas mixture these spectral intervals can be slightly extended. Emission in this spectrum and even far beyond can be achieved with the semiconductor based quantum cascade lasers. The benefits of these lasers are twofold: 1) the emitted spectrum lies in one of the atmospheric windows, i.e. 8-12 μm interval and 2) many materials absorb in this spectral region. The first benefit is important for the development of applications such as free space telecom systems and remote sensing (monitoring of atmospheric pollution e.g.). The second benefit is important in general spectroscopy (R&D and quality control in production processes) and for material processing in industrial environments as this spectral region is absorbed by many materials (metals, plastics, ceramics, silicon, polymers . . . ), for high-resolution laser assisted material processing such as cutting, welding, hole burning, marking, engraving, etc. and in the medical industry as biomaterials such as the skin do absorb these wavelengths as well.
The range and importance of laser assisted material processing in modern manufacturing is expanding at an impressive rate across many sectors in industry. Laser assisted material processing is inherently contact free. As such the problem of rapid wearing mechanical processing tools can be drastically reduced. Generally, the trend in pulsed laser assisted material processing is to use short pulses with high peak power in order to improve the edge quality. The high laser beam intensity provided by short pulse laser technology results in the vaporization-dominated material removal rather than the melt-expulsion-dominated mechanisms using longer duration pulses. This produces less thermal and mechanical shocks, less peripheral heat flow, what leads to reduced heat affected zones (HAZ) and less burn formation and hence more precise material removal. Just as important the short pulse duration produces very high peak power. This high peak power allows the laser to process difficult materials such as ceramics, etc. Due to the Q-switching mode, the peak power can be much higher than the CW power, meaning that much smaller lasers can be built to produce very high optical powers. Smaller lasers mean lower cost of ownership. Another advantage of such compact laser is the possibility to mount them directly on robotic arms.
When short powerful laser pulses can be provided at a high repetition rate, precision laser based material processing can be drastically speed up. Applications which can seriously benefit from it: the drilling of numerous small holes in paper or plastic parts without charring the edges of the paper or plastic material. Some examples are in the tobacco filtration, and in the banking and billing industries for perforating checks and other financial documents.
The existing solutions for producing short pulses is the on/off switching of the RF power of transversely excited atmospheric pressure (TEA) lasers and Q-switched CO2 lasers based on the electro-optic (E-O) or Pockels effect. The peak power and the efficiency of pulses TEA lasers are limited and their pulse repetition rates have an upper limit of about 500 Hz. The existing Q-switching CO2 lasers make use of the electro-optic effect: a long E-O crystal is needed due to the low electro-optic coefficient of e.g. “Cadmium Telluride” (n3r4l=1.09×10−10 m/V for λ=10.6 μm); high voltages (more than 5 kV for CO2 laser wavelengths) are required to change the output characteristics; Pockels modulators also need extra polarization sensitive devices inside the cavity, which is not a cost effective solution. The E-O crystal has an aperture at least larger than the laser beam. As this crystal has a wide aperture it is difficult to cool the central part, which for Gaussian-like beams is, however, the hottest part of the crystal. The most widely used electro-optic crystal in the spectral region of CO2 lasers is CdTe. This material features a substantial residual radiation absorption, which means that the optical intensity which may be incident on the crystal needs to be limited, the crystal is fragile and it is generally difficult to get anti-reflection coatings to adhere well to the entrance and exciting surfaces of the CdTe modulator crystals. These films can easily be damaged when inserted into CO2 laser feedback cavities. Anti-reflection coatings are used to reduce optical losses when these crystals are inserted within a laser feedback cavity to switch the cavity losses from a higher loss condition (i.e. low cavity Q) to a low loss condition (i.e. high cavity Q). Peeling and optical damaging of these coatings by the intense laser radiation is a common damage failure for these modulators when used to Q-switch CO2 lasers. Other modulation principles could be considered as acousto-optical (AO) or mechanical solutions. The relatively low diffraction efficiency of AOMs makes them not suited for Q-switching applications in the spectral range of a CO2 laser. Furthermore, AO-modulators are characterized by a trade-off between diffraction efficiency and damage threshold, which is a strong restriction with respect the power handling capabilities of these devices. Furthermore, the switching time is limited by the acoustic transit time through the crystal. Mechanical choppers are also characterized by relatively large switching times as compared to electro-optical solutions. Furthermore, this modulation principle is not flexible in the sense that the switching time is typically directly coupled to the rotational frequency and geometrical dimensions of the chopper blade and laser beam.
Hence, there is a need in the market of CO2 laser material processing for compact, low driving power high-rep rate efficient radiation modulators with a long lifetime for Q-switching CO2 lasers, which improve performance and economics of existing applications. It is desirable to make the Q-switched CO2 laser lower in cost, more reliable than the present state of the art of CdTe electro-optics crystal technology without sacrificing Q-switching performance, and obtaining higher peak power and shorter pulses.
It is also known to use absorption of radiation for detecting certain physical phenomena. Numerous detection principles based on absorption of radiation are known. Detection of certain phenomena is then based on the change such phenomena have on the absorption state in the detector. Typically, such detection is performed by monitoring the absorption of a radiation beam and by coupling variation of the absorption to the occurrence of a phenomenon, e.g. a physical phenomenon. Although several absorption-based detectors have been disclosed, there is still room for a reliable detector allowing sensing of physical phenomena which influence the state of absorption of an absorption-based detector.